Hemoglobin (Hb) A2, Quantitative

Hemoglobin A₂ levels have special application to the diagnosis of β-thalassemia trait, which may be present even though peripheral blood smear is normal. (This reflects the underlying genetic spectrum of β-thalassemia, which in reality is a complex of 20 to 30 distinct conditions.) The microcytosis and other morphologic changes of beta-thalassemia trait must be differentiated from iron deficiency. Low MCV may include the majority of β-thalassemia trait patients but does not differentiate iron deficient individuals. Low Hb A₂ levels occur in untreated iron deficiency. If the β-thalassemia is associated with iron deficiency, the Hb A₂ level falls, making the differentiation even more difficult (corrected after iron therapy).¹ The most definitive evidence for presence of β-thalassemia trait is genetic (family study). Offspring of a person with thalassemia major will have beta-thalassemia trait. Hb A₂ may be increased in megaloblastic anemia and may be decreased in sideroblastic anemia, Hb H disease, and erythroleukemia.